Because liquid crystal polymers (LCPs) have excellent moisture barrier properties, electronic components that are completely encased in a molded LCP package are hermetically insulated from exposure to atmospheric moisture. However, if the package has electrical leads that enter or exit the package for purposes of connecting with exterior components, these electronic ports become points for moisture entry and hermeticity is compromised.
Air cavity packages are widely used and are a chip containment system consisting of a structural base, a die attach pad, lead frame and lid. After die and wire bonding to the base, the lid is affixed to provide environmental and mechanical protection for the die and wire bonds without touching either of these components. Applications that require hermetic packages have historically used ceramic or metal for the base and lid. Thermoplastics had not had the moisture barrier properties to achieve hermeticity.
Recently, a new class of thermoplastics called Liquid Crystal Polymers (LCP), have been introduced with excellent moisture barrier properties. Over the last several years, there has been a push to develop technology to use molded LCP in hermetic packages primarily because molded thermoplastics are substantially cheaper than ceramic or metal hermetic packages.
Hermetic electrical ports in LCP packages have long presented a technical challenge. Although molded LCP has excellent moisture barrier properties, design of an electrical port such that metal leads can enter and exit without compromising hermeticity is challenging primarily because adhesion between LCP and metal is not good. LCPs are chemically inert, which is desirable in many applications, but is an impediment in achieving good adhesion to other materials. Attempts have been made to over-mold LCP on metal pins, for example to a lead frame comprising a pair of spaced apart parallel busses with intervening components connected via leads much like the rungs of a ladder. RJR Polymers' LCP molding operation uses a patented method to seal leads from the lead frame as they pass through the sidewalls of the LCP package. However, it involves placing a small amount of epoxy on the frame during molding to. form a strong metal-to-LCP bond. The epoxy bead becomes a path for moisture ingress over time and temperature.
LCP resins are commercially available from several major suppliers—Ticona, Allied Chemicals, Dupont and Sumitomo. LCPs have the following advantages:                (a) Extremely low moisture absorption and transmission. No hydrolysis problem even at elevated temperatures.        (b) Excellent chemical stability—no effect of immersion for prolonged periods in organic solvents, sulfuric acid, chromic acid, aviation fuels.        (c) No thermal degradation up to 450° C. (Jin, 1999).        (d) Tensile strength comparable to Kevlar.        
LCPs exhibit the lowest moisture transmission and absorption rates of all polymers. The permeability of LCPs to oxygen is also extremely low. As a consequence, these materials are used extensively in the food packaging industry, where thin layers of LCP are co-extruded with more conventional polymers such as polyethylene to provide an effective barrier to oxygen. LCPs are now being used to package delicate electronic, electro-optic and MEMs devices, since they present a cost effective alternative to more conventional hermetic packaging based on metal, glass and ceramic.
Accordingly, it is a primary object of the present invention to provide methods of fabricating hermetic electrical ports in molded LCP packages for electronic components and circuits.
It is yet another object of the present invention to provide LCP packages for electronic components and circuits having hermetic electrical ports for electrically connecting with exterior components.
Other objects of the invention will, in part, be obvious, and will, in part, appear hereinafter when the following detailed description is read in connection with the accompanying drawings.